1. Field of the Invention
The present invention relates generally to methods and devices for the nondestructive evaluation of materials. The present invention relates more specifically to a magnetostrictive sensor based system for the inspection of pipeline structures from an in-line inspection vehicle.
2. Description of the Related Art
The use of magnetostrictive sensors (MsS) in the nondestructive evaluation (NDE) of materials has proven to be very effective in characterizing defects, inclusions, and corrosion within various types of ferromagnetic and non-ferromagnetic structures. A MsS launches a short duration (or a pulse) of elastic guided waves in the structure under investigation and detects guided wave signals reflected from anomalies such as defects in the structure. Since guided waves can propagate long distances (typically 100 ft or more), the MsS technique can inspect a significant volume of a structure very quickly. In comparison, other conventional NDE techniques such as ultrasonics and eddy current inspect only the local area immediately adjacent to the probes used. Therefore, the use of magnetostrictive sensors offers a very cost effective means for inspecting large areas of steel structures such as strands, cables, pipes, and tubes quickly with minimum support requirements such as surface preparation, scaffolding, and insulation removal. The ability to use magnetostrictive sensors with little preparation of the object under inspection derives from the fact that direct physical contact between the sensors and the material is not required.
Efforts have been made in the past to utilize magnetostrictive sensor technologies in association with the inspection of both ferromagnetic and non-ferromagnetic materials. Included in these efforts are systems described in U.S. Pat. Nos. 5,456,113, 5,457,994 and 5,501,037 which are each commonly owned by the assignee of the present invention. The disclosures of U.S. Pat. Nos. 5,456,113, 5,457,1994 and 5,501,037 provide background on the magnetostrictive effect and its use in NDE and are therefor incorporated herein by reference. These efforts in the past have focused primarily on the external inspection of piping, tubing and steel strands/cables wherein the nature of the structure is such that uninterrupted internal access to the pipe wall is very limited. While these systems and their external application to longitudinal structures find significant applications, there are yet other inspection techniques structures that could benefit from the use of magnetostrictive based NDE.
The nondestructive evaluation of materials using magnetostrictive sensors is based upon the magnetostrictive effect and its inverse effect. The magnetostrictive effect is a phenomenon that causes the physical dimensions of a ferromagnetic material to change slightly when the material is magnetized or demagnetized or otherwise experiences a changing magnetic field. The inverse effect is a phenomenon that causes a magnetic flux in the material to change when the material is stressed. Systems utilizing magnetostrictive sensors use the magnetostrictive effect and its inverse effect to generate and detect guided waves that travel through the ferromagnetic material.
In general, a magnetostrictive sensor consists of a conductive coil and a means for providing a DC bias magnetic field in the structure under inspection. The means for providing a bias magnetic field can include the use of either permanent magnets or electromagnets. In a transmitting magnetostrictive sensor, an AC electric current pulse is applied to the coil. The resulting AC magnetic field (a changing magnetic field) produces elastic waves (also known as guided waves) in an adjacent ferromagnetic material through the magnetostrictive effect. In the receiving magnetostrictive sensor, a responsive electric voltage signal is produced in the conductive coil when the elastic waves (transmitted or reflected from anomalies within the material) pass the sensor location, through the inverse magnetostrictive effect.
With MsS techniques, defects are typically detected by using the pulse-echo method well known in the field of ultrasonics. Since the sensor relies on the magnetostrictive behavior found in ferromagnetic materials, this technology is primarily applicable to the inspection of ferromagnetic components such as carbon steel piping or steel strands. It is also applicable, however, to the inspection of nonferrous components if a thin layer of ferromagnetic material, such as nickel, is plated or coupled onto the component in the area adjacent to the magnetostrictive sensors.
The magnetostrictive sensor technique has the advantage of being able to inspect a large area of material from a single sensor location. Such sensors have, for example, been used to accurately inspect a length of pipe or cable of significantly more than 100 feet. Further, magnetostrictive sensor techniques are comprehensive in their inspection in that the methods can detect both internal and external defects, thereby providing a 100% volumetric inspection. The techniques are also quite sensitive, being capable of detecting a defect with a cross-section less than 1% of the total metallic cross-section of cylindrical structures such as pipes, tubes, or rods. Finally, as indicated above, magnetostrictive sensor techniques do not require direct physical contact between the component surface and the sensor itself. This eliminates the need for surface preparation and permits the movement of the sensor across the surface without concern for abrasive contact.
Gas transmission pipelines are typical of tubular structures that regularly require inspection for defects to insure their structural integrity and their safe operation. The primary traditional tool utilized to inspect such pipelines is referred to an in-line inspection (ILI) vehicle or pig that is equipped with an inspection device and travels down the length of the pipeline inside the conduit. The detection of corrosion metal loss is typically accomplished using devices based on the magnetic flux leakage (MFL) technique. Magnetic flux leakage devices work well, although they are heavy and difficult to handle. In most instances, MFL devices lack the flexibility to accommodate different pipe diameters, and as such different devices are needed for each pipeline diameter to be inspected.
For the detection of cracks such as stress corrosion cracking (SCC) that occur in the longitudinal direction of a pipeline, devices based on ultrasonic techniques are frequently used. Ultrasonic devices, such as those developed by British Gas, employ an array of wheel type piezoelectric transducers to couple an ultrasonic wave into and out from the pipe wall without the need of a liquid couplant. Such ultrasonic devices work reasonably well but tend to be very expensive to build and operate. Because of the high inspection costs associated with ultrasonic devices, the gas pipeline industry has devoted much research to finding a more economical approach to pipeline inspection.
One current direction of the active research and development in the gas pipeline industry focuses on the use of electromagnetic acoustic transducers (EMATs) which require no liquid couplant to convey a signal to and from the investigated material. Other research and development efforts are focusing on systems that use the high-pressure gas as a coupling medium to convey the interrogating signal. Recent applications of plate magnetostrictive sensor probes have shown promise in a variety of structural geometrys. In addition to the benefits associated with not requiring a liquid couplant, magnetostrictive sensor probes offer further advantages in that: (1) they can detect both corrosion metal loss and stress-corrosion cracking as well as coating disbond; (2) they are simple in design, lightweight, and easy to handle; (3) they can readily accommodate different pipeline diameters; and (4) they are economical to manufacture and operate.
A plate magnetostrictive sensor operates by using the magnetostrictive force as described above and thus differs from EMATs which are based on the Lorentz force. EMATs used on ferromagnetic steel also encounter the magnetostrictive force and can utilize the magnetostrictive force for wave generation and detection. However, EMATs use a meandering coil type design where the adjacent coil lines are separated by a half wave length distance in order to reinforce a localized excitation and detection in the material. In order to maintain a reasonable sensor size, EMATs are designed to operate at relatively high frequency (typically over 500 kHz). A few EMAT sensors have been developed that are capable of operating down to about 250 kHz.
Plate type magnetostrictive sensor probes are designed quite differently from EMAT based sensors. Plate type magnetostrictive sensors consist of a coil wound on a U-shaped core. Typically the coil is 50 to 100 turns and the U-shaped core is 6 to 10 inches long. The plate type magnetostrictive sensor probes typically operate below 200 kHz. Because of the unique sensor design and low frequency operation, the magnetostrictive probes have good sensitivity, are more tolerant to lift off, and have a longer inspection range than generally available EMATs.
Efforts at providing methods and devices for detecting defects in pipelines have included the following:
U.S. Pat. No. 5,907,100 issued to Cook on May 25, 1999 entitled Method and System for Detecting and Displaying Defects in Piping. This patent describes a typical EMAT sensor based device wherein an EMAT transmitter sends an ultrasonic wave through the pipe wall and receives a reflected ultrasonic signal from a defect in the pipe.
U.S. Pat. No. 5,675,251 issued to MacLean et al. on Oct. 7, 1997 entitled Device and Method for Inspection of Pipelines. This patent describes a plurality of housing units that are generally spherical in shape for inspecting the integrity of water distribution pipelines. The housing units are connected by flexible connections that permit movement of the inspection device easily through bends and constricted areas within the pipeline.
U.S. Pat. No. 4,439,730 issued to Kaufmann on Mar. 27, 1984 entitled Nondestructive Inspection Apparatus and Method Utilizing Combined Inspection Signals Obtained from Orthogonal Magnetic Fields. This patent describes one of the above mentioned magnetic flux leakage (MFL) techniques currently utilized in conjunction with in-line inspection vehicles. The process described involves establishing a steady magnetic flux field through an area in a first direction and then passing a magnetic flux field through the same area in an orthogonal direction.
U.S. Pat. No. 5,454,276 issued to Wernicke on Oct. 3, 1995 entitled Multidirectional Magnetic Flux Pipe Inspection Apparatus and Method. This patent likewise describes a magnetic flux leakage (MFL) technique device positioned on a pipeline inspection pig having a drive mechanism and magnetic field generators. The method anticipates a helical progression through the pipeline so as to generate a grid of helical sensor signals from a plurality of MFL sensors.
U.S. Pat. No. 5,864,232 issued to Laursen on Jan. 26, 1999 entitled Magnetic Flux Pipe Inspection Apparatus for Analyzing Anomalies in a Pipeline Wall. This patent describes yet another magnetic flux leakage technique device whose functional principles are schematically explained in FIG. 2 of the patent. A variety of mechanisms for maintaining the sensors in close proximity to the pipeline wall are described. Wear plates and wear pads mounted on the top of the sensor body are described for reducing wear on the contacting sensor.
U.S. Pat. No. 5,581,037 issued to Kwun et al. on Dec. 3, 1996 entitled Nondestructive Evaluation of Pipes and Tubes Using Magnetostrictive Sensors. This patent describes an application of magnetostrictive technologies and techniques as applied externally on a longitudinal body such as a pipe tube or other cylindrical shell. The system anticipates the establishment of longitudinally directed mechanical waves within the pipeline which are detected at distant magnetostrictive sensors.
U.S. Pat. No. 6,023,986 issued to Smith et al. on Feb. 15, 2000 entitled Magnetostrictive Flux Leakage Inspection Technique for Pipelines. This patent describes yet another MFL system that includes an inertial navigation system and a global positioning system. The basic structure for establishing a magnetic flux leakage sensor is shown in FIG. 1 of the patent where magnetic coupling to the wall of the pipeline is made by way of pliable steel brushes.
As indicated above, it would be desirable to benefit from the advantages that magnetostrictive sensor probes have over EMAT and MFL based sensor probes within a structure and with a technique that traverses the internal space of a gas pipeline for inspection purposes. It would be advantageous to be able to utilize plate magnetostrictive sensor probes as are currently externally applied to materials under investigation to an in-line transport device capable of moving the sensors down the length of a gas pipeline or the like.
It would therefore be desirable to implement magnetostrictive sensor techniques in conjunction with pipeline structures in a manner similar to, and with the accuracy of, such systems utilized externally in conjunction longitudinal cylindrical structures. It would be desirable if an inspection of pipeline structures could be carried out in an efficient manner that did not require access to the outside surface of the pipeline. Such a magnetostrictive sensor system would be able to investigate long lengths of a pipeline structure and would provide a cost effective global inspection of the structure.
It is therefore an object of the present invention to provide a sensor device for implementing magnetostrictive based NDE in association with pipeline structures in order to evaluate the condition of the structures and determine the presence of anomalies indicative of fractures, deteriorations, and the like.
It is a further object of the present invention to provide magnetostrictive sensor devices appropriate for use in conjunction with the inspection of pipeline structures that is capable of transmitting and receiving guided waves within the pipeline wall structures and generating signals representative of the characteristics of such waves appropriate for the analysis and detection of anomalies therein.
It is a further object of the present invention to provide magnetostrictive sensor devices appropriate for use in conjunction with the inspection of pipeline structures that progressively inspect the circumference of the pipeline structure for anomalies, corrosion, fractures, and the like in a cost effective manner.
It is a further object of the present invention to provide a method for the inspection of pipeline structures that includes the use of a magnetostrictive sensor specifically adapted for directing guided waves into the pipeline wall and detecting such waves as may be reflected from anomalies within the pipeline wall.
It is a further object of the present invention to provide a method and apparatus for the nondestructive evaluation of pipeline structures utilizing magnetostrictive sensors that are capable of progressively investigating large volumes of the pipeline structures without access to the external surface area of the pipeline.
In fulfillment of these and other objectives, the present invention provides a method and system for implementing magnetostrictive sensor techniques for the nondestructive evaluation of pipeline structures. The system consists of a magnetostrictive sensor instrument unit, a data storage unit, and a plurality of magnetostrictive sensor probes that are positioned on an in-line inspection vehicle. The instrumentation unit includes electronics for transmitting excitation pulses to a transmitting magnetostrictive sensor probe as well as electronics for amplifying and conditioning the signals detected by a receiving magnetostrictive sensor probe. The magnetostrictive sensor probes include both plate magnetostrictive sensors and permanent magnets which provide a DC bias magnetic field necessary for magnetostrictive sensor operation. The transmitting and receiving probes are attached to the in-line inspection vehicle by way of mechanical arms on opposing sides of the vehicle. The mechanical arms are spring loaded and are equipped with rollers which maintain the probes at approximately constant distances from the inside diameter of the pipe wall. The method involves generating pulses of shear horizontal waves of frequencies less than 200 kHz. The transmitting magnetostrictive sensor probe generates a wave that propagates in both directions around the circumference of the pipe wall from a point adjacent to the transmitting probe. Both waves are thereafter received at the receiving probe spaced 180 degrees apart from the transmitting probe. Any defect present in the pipe wall within the circumference being investigated will show up in the received signal.